Stacked Rapid Sand Filter

Design information for the AguaClara Stacked Rapid Sand Filter (StaRS) is available in the Open Stacked Rapid Sand Filter Design chapter of The Physics of Water Treatment Design

Purpose and Description

The StaRS filter removes particles and pathogens from the clarified water. The StaRS filter was designed to eliminate the need for pumps and storage tanks that are normally required for backwashing rapid sand filters. The StaRS filter has the same flow rate in filtration mode and in backwash mode. An elegant filter hydraulic design enables switching from filtration mode to backwash mode without using any large diameter valves. Backwash is accomplished using clarified water and that increases the efficiency of water production by the AguaClara plant.

The hydraulic controls are on the main plant floor and are open so that the plant operators can readily observe head loss through the filter as well as the clarity of the clarified and filtered water.

The plan view of the filter (Fig. 22) shows the filter chambers and the hydraulic controls separated by a walkway.

Fig. 22 Filter top view showing filter hydraulic controls and filter chambers for a 2 filter chamber design.

The isometric side view of the filter (Fig. 23) with the filter wall and backwash tank wall removed shows the filter internal piping as well as the pipes that connect the filter controls to the filter chamber.

Fig. 23 Filter isometric view showing filter hydraulic controls and filter chambers for a 2 filter chamber design.

The section view cutting through the center of the first filter chamber (Fig. 24) shows that the center of the filter controls inlet tank is aligned with the center of the filter chamber. Thus the four inlet pipes have a straight path into the filter chamber. The outlet pipes have a 45° elbow to account for the offset to the filter controls outlet tank.

Fig. 24 Filter left section isometric view showing how the inlet tank, filter chamber, and siphon pipe are all aligned. The different heights of the inlet pipe stubs in the inlet tank are also shown.

The filter controls (Fig. 25) provide the operator with a method to rapidly confirm filter operation. The filter controls come in pairs when there is more than one filter chamber so they can share the finished water tank.

Fig. 25 Filter controls top isometric view.

Water flows from right to left from the inlet channel, over the wide weir to the bypass channel. During normal operation the water enters the LFOM and flows into the filter controls inlet tank (Fig. 26). If there is a need to bypass a filter the LFOM and bypass pipe stubs are simply swapped. During backwash the gate can be removed so that the filter receives the design flow rate. The filter controls inlet tank has an overflow that dumps water to the backwash tank when the head loss through the filter exceeds the maximum design value. The bubble weir raises bubbles to minimize their entry into the filter inlets. The short pipe stubs in the filter inlets ensure that the water all enters the backwash inlet during backwash. The different heights of the pipe stubs causes the filter sand to fluidize gradually starting from the sand near the top of the filter and progressing deeper as more of the water is forced into the bottom inlets.

Fig. 26 Filter controls inlet tank. The backwash orifice plate is in place for filtration mode.

Water exits the filter chamber through three outlet pipes and then enters the filter controls outlet tank (Fig. 27). After backwash the red pipe stub can be removed as shown in the foreground outlet tank to enable filtering to waste. When not filtering to waste the water flows over the filtered water weir into the finished water tank and finally into the finished water pipe.

Fig. 27 Filter controls outlet tank section showing water levels. The outlet tank in the foreground has the pipe stub that enables filter to waste removed. The outlet tank in the background has the pipe stub in place to prevent filtering to waste.

The filter chamber internal piping (Fig. 28) serves to divide the sand bed into 6 layers. The with flow through each layer alternates direction with the bottom layer flowing upward and the next layer flowing downward.

Fig. 28 Filter internal piping showing how the modules, siphon, and filter drain are placed in the filter chamber.

The filter internal piping consists of 7 modules (Fig. 29) for easy assembly in the filter chamber. Each module stacks on top of the module below. The tie-down cable ensures that the modules can not shift relative to each other and it also clamps the entire set of modules in place to prevent uplift during backwash initiation.

Fig. 29 Filter internal piping modules showing how the modules stack, are prevented from lifting by cables anchored to the concrete slab, and attach to the pipes embedded in the filter chamber wall.

The main trunk pipe (Fig. 30) in each module is connected to the embedded pipe that connects to either the filter control inlet tank or filter control outlet tank. The pipe connections is a stainless steel band that is then held in place with two hose clamps. The ports for the branches slightly smaller than the outside diameter of the branches so that the molded end of the branches can only be inserted a fixed distance into the trunk.

Fig. 30 Filter inlet trunk isometric showing the ports for the branches and the band clamp system that attaches to a pipe embedded in the filter chamber wall.

The 200 mm spacing of the trunks and receptors is set by the spacers (Fig. 31). The cable grooves alternate sides on the spacers so that the cables prevent the spacers from moving. The spacers are attached to the trunks and receptors.

Fig. 31 Filter trunk spacer front and top views.

The receptors (Fig. 32) are symmetrical and have PVC disks that plug both ends. The compact design of the PVC disks enables the first and last branches to be relatively close to the filter chamber walls.

Fig. 32 Filter inlet receptor isometric showing the end plug and the ports for the branches.

There are three types of inlet branches (Fig. 33). The bottom backwash branches have the most orifices to be able to handle the high backwash flow rate. This results in the backwash branches having insufficient head loss during normal filtration. To accommodate the low backwash branch head loss an orifice plate is added to the backwash inlet during filtration. The middle two inlet modules both deliver clarified water to two sand layers and thus should have at least twice as many orifices as the top inlet branches. The middle inlets need even more orifices to account for the added head loss in the inner trunks given that the inner trunks have double the flow rate of the top and bottom trunks during filtration.

Fig. 33 Filter inlet branches showing the bottom view (top to bottom) of the backwash branches, middle two inlet branches, top inlet branches, and side view of an inlet branch. The ends of the branches are molded to create a sand tight connection with the trunk and receptor pipes.

Filter Keys

Table 16 Distribution to the filter inlet tanks.

Key	Name	Purpose
1	inlet channel	receives the water from 1 clarified water pipe coming from the clarifier and distributes the water to 1 filter chambers
2	wide weir	uses head loss to divide the flow equally between the filter chambers
3	removable gate	can be removed to provide full design flow to a filter during backwash
4	bypass channel	provides option to bypass the filter by swapping the LFOM and bypass pipes
5	LFOM	measures the flow rate into the filter
6	bypass pipe	pipe stub that can be swapped with the lfom to bypass the filter

Table 17 Filter inlet tank.

Key	Name	Purpose
7	inlet tank	distributes the clarified water to the 4 filter inlets
8	overflow	automatically diverts clarified water to the pipe gallery when the filter head loss exceeds the maximum design value
9	bubble weir	lifts bubbles to prevent them from entering the inlets to the filter
10	backwash orifice plate	adds head loss to filter inlet 1 during filtration so that it has the same head loss as the other inlets. Removed during backwash.
11	backwash trunk	deliver water from the inlet tank to the filter chamber during filtration and during backwash
12	inlet trunks	deliver water from the inlet tank to the filter chamber during filtration

Table 18 Filter outlet tank.

Key	Name	Purpose
13	outlet tank	collects filtered water from the 3 outlet trunks
14	outlet trunks	deliver water from the filter chamber to the outlet tank during filtration
15	filter to waste	dumps filtered water to the pipe gallery. Remove the pipe stub after backwash to waste filtered water that doesn’t meet treatment standards.
16	filtered water weir	enables filtering to waste
17	finished water tank	adds chlorine to the filtered water and delivers the water to the finished water pipe
18	chlorination conduit	provides a path for a chlorination tube to drip into the water exiting from one of the filters
19	finished water pipe	delivers the finished water to the community water storage tank

Table 19 Filter chamber and siphon.

Key	Name	Purpose
20	filter chamber	contains the filter internal piping and filtered sand
21	filter chamber drain	drains water for maintenance operations
22	sand dump pipe	empties the sand from the filter
23	siphon pipe	discharges backwash to the pipe gallery
24	siphon air valve	controls the siphon and switches the filter from filtration to backwash and back again
25	siphon water seal	prevents the water from exiting the siphon at the end of the backwash
26	sand	provides the filter media and pore structure for capture of flocs and particles

Table 20 Filter internal piping.

Key	Name	Purpose
27	band clamps	attach the trunks to the pipes that are embedded in the filter tank chamber wall
28	winged branch pipes	injects clarified water into the sand bed
29	slotted branch pipes	extracts filtered water from the sand bed
30	receptor pipes	terminate and support the winged and slotted branch pipes
31	trunk spacer	supports the end of the trunks
32	receptor spacer	supports the receptor spacers
33	anchor bolts	connects the cable to the concrete slab
34	cable	prevents the internal pipe modules from lifting during backwash initation
35	hose clamp	tightens all of the connections between branches, receptors, and trunks to prevent sand leaks

Table 21 Filter appurtenances.

Key	Name	Purpose
36	sand tank	receives, drains, and temporarily stores filter sand during filter maintenance operations
37	sand tank overflow weir	excess water discharges over this weir
38	slotted sand drain	discharges water from the sand
39	sand tank overflow drain	discharges overflow water from the sand tank
40	pipe gallery and filter backwash water tank	option to use this tank to enable recycle of backwash water
41	pipe gallery drain	discharges water from the pipe gallery

Filter Design Parameters

Table 22 Distribution to the filter inlet tanks design parameters.

Key	Name	Value
1	inlet channel
	water depth	1.23 m
	width	460 mm
2	wide weir
	height	400 mm
	width	580 mm
	maximum head loss	50 mm
3	removable gate
	height	200 mm
	width	220 mm
4	bypass channel
	width	280 mm
5	LFOM
	nominal diameter	6 inch
	SDR	41
	number of rows of orifices	10
	maximum flow rate	12 L/s
	head loss at maximum flow	200 mm
	diameter of orifices	19.1 mm
	space between orifices measured on the outside of the pipe	3 mm
	orifices in each row starting from bottom row	[ 15, 4, 5, 3, 3, 4, 2, 2, 3, 2]
	elevation of each row from zero flow datum	[ 9.53 mm, 29.6 mm, 49.7 mm, 69.8 mm, 89.9 mm, 110 mm, 130 mm, 150 mm, 170 mm, 190 mm]
6	bypass pipe
	nominal diameter	6 inch
	SDR	41

Table 23 Inlet tanks design parameters.

Key	Name	Value
7	filter inlet tank
	length	1.44 m
	width	460 mm
8	overflow
	nominal diameter	4 inch
	SDR	26
	maximum water depth	980 mm
9	bubble weir
	height	190 mm
	width	460 mm
10	backwash orifice plate
	orifice diameter	60 mm
11	backwash trunk
	nominal diameter	6 inch
	SDR	41
	maximum head loss during filtration	34.6 mm
	maximum velocity during filtration	99.4 mm/s
	maximum head loss during backwash	156 mm
	maximum velocity during filtration	596 mm/s
12	inlet trunks
	nominal diameter	6 inch
	SDR	41
	maximum head loss during filtration	34.6 mm
	inner trunks maximum velocity during filtration	199 mm/s
	inlet 1 (backwash) pipe stub height	none
	inlet 2 pipe stub height above the slab	100 mm
	inlet 3 pipe stub height above the slab	1.25 * 100 mm
	inlet 4 pipe stub height above the slab	1.5 * 100 mm

Table 24 Outlet tanks design parameters.

Key	Name	Value
13	outlet tank
	length	460 mm
	width	280 mm
14	outlet trunks
	nominal diameter	6 inch
	SDR	41 inch
15	filter to waste
	nominal diameter	4 inch
	SDR	26
16	filtered water weir
	height	100 mm
	maximum head loss	58.2 mm
17	finished water tank
	half width	115 mm
18	chlorination conduit
	nominal diameter	3 inch
	SDR	26
19	finished water pipe
	nominal diameter	4 inch
	SDR	26

Table 25 Filter chambers design parameters.

Key	Name	Value
20	filter chamber
	number of filter chambers	1
	number of spare filter chambers	0
	length	900 mm
	width	1.44 m
	height	4.61 m
	number of stacked filters	6
	height of each layer	200 mm
21	filter chamber drain
	nominal diameter	1.5 inch
22	sand dump pipe
	nominal diameter	2 inch
	SDR	26
	Discharge height	1 m
23	siphon pipe
	nominal diameter	6 inch
	SDR	41
	initial flow rate at beginning of backwash	61.2 L/s
	head loss at filter chamber design flow	95.4 mm
	number of orifices	15
	orifice diameter	42.4 mm
	orifice center to center spacing	49.9 mm
24	siphon air valve
	nominal diameter siphon control air vent valve	0.5
25	siphon water seal
	nominal diameter siphon control air vent valve	10
	head loss over the weir of the pipe	39.7 mm
	optional concrete fill height	1.66 m
26	sand
	depth	1.34 m
	fluidized depth	1.25 m
	density	2.65 kg/L
	porosity	0.4
	effective size	500 μm
	clean bed headloss at 5 °C	75.4 mm
	clean bed headloss at 25 °C	45.4 mm
	head loss to fluidize sand	1.24 m
	bulk volume of sand per filter chamber (not accounting for volume of internal pipes)	1.73 kL
	mass of sand per filter chamber (not accounting for volume of internal pipes)	1.84 Mg

Table 26 Filter internal piping design parameters.

Key	Name	Value
27	band clamps
	band width	40 mm
	band thickness	200 μm
28	winged branch pipes
	nominal diameter	1.5 inch
	SDR	12.3
29	slotted branch pipes
	nominal diameter	1.5 inch
	SDR	12.3
	length (not including molded ends)	502 mm
30	receptor pipes
	nominal diameter	3 inch
	SDR	26
31	trunk spacer
	thickness	25.4 mm
32	receptor spacer
	thickness	25.4 mm
33	anchor bolts
	maximum force on anchor bolts	1750 { kilogram : 1 , meter : 1 , second : -2 }
34	cable
	diameter	4.76 mm
	maximum force on trunk cables	1750 { kilogram : 1 , meter : 1 , second : -2 }
35	hose clamp

Table 27 Filter appurtenances design parameters.

Key	Name	Purpose
36	sand tank
	minimum volume	1.73 kL
	length	2.83 m
	width	1.72 m
37	sand tank overflow weir
	height	610 mm
38	slotted sand drain
	nominal diameter	1.5 inch
39	sand tank overflow drain
	nominal diameter	4 inch
40	pipe gallery and filter backwash water tank
	maximum depth	1.88 m
	volume	20.7 kL
	maximum number of backwash cycles	20.7 kL
41	pipe gallery drain
	nominal diameter	4 inch